FLOATING AND RAFTING 389 
not be possible to drive logs more than a few miles at most before 
the force of the water is spent. In such cases frequent storage 
basins are required. 
(c) The length of time for which flood water must be available. 
If artificial freshets are required only for a short time in the 
spring when the streams are fed from snow water a smaller stor- 
age area may be used than when water must be available for 
several months. 
DAMS 
Dams for logging purposes are usually built of round timber 
secured close to the dam site. 
It is necessary to construct a dam on solid bottom or bed 
rock because if this is not done water will work underneath the 
sills and ultimately cause the structure to go out. 
There are three types of timber dams used for logging pur- 
poses: (1) the crib or pier dam, (2) the rafter or self-loading 
dam, (3) the pile dam. 
Concrete dams of large size are occasionally used by lumber 
companies, but they are built by engineers, and loggers are 
seldom concerned in their construction. 
Timber dams on small streams usually have a sluiceway 
through which logs are run and waste water passed, while on 
large streams several waste gates are required to take care of 
surplus water. "Roll dams' which have no gates or sluice- 
ways are also built to raise the stream level. The water and 
logs pass over the crest of the dam. 
The choice of the type of dam to be used depends upon : 
(1) The character of the bottoms. When the subsoil is un- 
stable, the dam should be of a type which rests upon solid founda- 
tion; otherwise the structure will be undermined and carried 
away. 
(2) The head of water desired. The water pressure against 
the dam increases with the height of the head of water carried, 
therefore, the construction must be stronger as the height increases. 
Dams are subject chiefly to three forces which cause them to 
become dislodged and carried away, namely crushing, sliding 
and overthrow. These are due to the pressure of the water on 
the upstream face of the structure. The crushing stress is over- 
come by making the timbers of ample size to resist this pressure, 
